Use changedTouches instead of targetTouches in style.getClientPosition

`targetTouches` is not defined for `touchend` or `touchcancel` events. `changedTouches` is always defined.

With the current implementation (after 116609a6de5a687a85299f3feeb6e044efa0d1a5) the position is always `[0, 0]` 

closes #41